It is commonly thought that God’s existence is a matter of faith alone, not reason. Yet the Catholic Church teaches that God’s existence can be proved by reason alone.

A philosophy degree from a state school offers a lot more than just job security. It also provides a host of ingenious linguistic stratagems for denying God’s existence — anything from the silly paradox “Can God create a rock so heavy he can’t lift it?” to the obnoxious conundrum “If God created everything, who created God?” As an undergraduate philosophy student I became used to hearing — from students and faculty alike — that the question of God’s existence is virtually unsolvable by reason and belongs to the domain of faith alone.

Has Reason Nothing to Say?

It has become such a mainstay of contemporary philosophical orthodoxy that the matter of God’s existence belongs to faith alone, that even Catholics have fallen prey to the relentless barrage of skepticism in this respect. It seems to be taken for granted that we cannot prove God’s existence. It is assumed that we cannot know; we can only have faith.

Is this a warranted assumption? Are we really incapable of knowing that God exists and of demonstrating by reason His existence? It is perhaps a little-known fact that the Catholic Church teaches that we can indeed know that God exists by reason alone. We find in the Dogmatic Constitution on the Catholic Faith (Dei Filius), of the First Vatican Council (1869-1870), the following: “The same Holy mother Church holds and teaches that God, the source and end of all things, can be known with certainty from the consideration of created things, by the natural power of human reason” (2.1). The document later says, “If anyone says that the one, true God, our creator and lord, cannot be known with certainty from the things that have been made, by the natural light of human reason: let him be anathema” (Canon 2.1). The Church bases her teaching on the words of St. Paul: “Ever since the creation of the world, his invisible attributes of eternal power and divinity have been able to be understood and perceived in what he has made” (Rom 1:20).

The Five Ways of St. Thomas Aquinas

The existence of God can be proved in five ways.

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion.… Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible.

The third way is taken from possibility and necessity, and runs thus. We find in nature things that are possible to be and not to be, since they are found to be generated, and to corrupt, and consequently, they are possible to be and not to be…. Therefore we cannot but postulate the existence of some being having of itself its own necessity, and not receiving it from another, but rather causing in others their necessity. This all men speak of as God.

The fourth way is taken from the gradation to be found in things. Among beings there are some more and some less good, true, noble and the like.… Therefore there must also be something which is to all beings the cause of their being, goodness and every other perfection; and this we call God.

The fifth way is taken from the governance of the world … some intelligent being exists by whom all natural things are directed to their end; and this being we call God.

— Summa Theologiae, Part I, Question 2, Article 3

Notice that the Church here teaches that we can know with certainty of God’s existence by the natural light of human reason. The Church says this with certainty lest we be under the impression that our knowledge is uncertain; and she says this by the natural power of human reason lest we think that the only possible source of this knowledge is faith and faith alone. We find in this teaching of the Church a resolute stance against skepticism and the vindication of the reason’s ability to know — and to know with certainty — the source and end of all things.

Historically, the discipline of philosophy has largely concurred in this position of the Church. It was really not until the 19th century that radical, atheistic skepticism became widely accepted and even mainstream positions in the field. Thinkers such as Ludwig Feuerbach, Karl Marx and Friedrich Nietzsche were just a few of the new, radical thinkers behind the “death of God” movement, still read and admired today by disaffected college students and (pseudo-)intellectuals. The fact is, metaphysics — the central branch of philosophy that studies the nature of being — has been predominantly theistic since the days of the great Greek philosophers Plato and Aristotle in the fourth century B.C. The idea that systematic reasoning (in essence what philosophy is) somehow inevitably excludes God is simply absurd.

Furthermore, the doctrine of fideism, which pits faith against reason and denies the role of reason in religious faith, was always condemned by the Catholic Church. Vatican I taught, as the Church had always taught, that faith and reason constitute two, mutually harmonious orders of knowledge: “Even though faith is above reason, there can never be any real disagreement between faith and reason, since it is the same God who reveals the mysteries and infuses faith, and who has endowed the human mind with the light of reason” (Dei Filius, 4.5). Faith reaches farther than reason and preserves reason from error, yet reason has a significant role to play. The old Catholic Encyclopedia, reiterating Vatican I as well as St. Thomas Aquinas, explains: “Reason should prepare the minds of men to receive the Faith by proving the truths which faith presupposes; reason should explain and develop the truths of faith and should propose them in scientific form; [and] reason should defend the truths revealed by God.”

The Five Ways

St. Thomas Aquinas, the Church’s theologian par excellence — whose nearly singular purpose was to illustrate this harmony between faith and reason — went so far as to posit five arguments for the existence of God. These comprise the traditional arguments for God’s existence advanced by Catholics since the time of St. Thomas in the 13th century, though the basis for much of the argumentation can be traced back to Aristotle some 1,700 years before him. These five ways are (1) the argument from motion; (2) from the nature of the efficient cause; (3) from possibility and necessity; (4) from the gradation of perfection found in things; and (5) from the governance of the world.

St. Thomas thought that the clearest way was the first, the argument from motion, so this is the argument upon which I’d like to focus here. This is also known as the cosmological argument, since it begins with the reality of motion in the world that is evident to our senses. In a somewhat simplified form, the argument runs as follows: we experience things in the world that are “in motion” — things that are changing (for St. Thomas, motion and change were basically the same thing). Whatever is in motion is put in motion by something else, and this is in turn moved by something else, and so on. But this cannot go on to infinity, because just as a staff moves only because it is put in motion by the hand, so too must there be a first mover put in motion by no other, or else nothing could move at all. This first mover is what everyone understands to be God (see Summa Theologiae I, Q. 2, Art. 3). This argument, as all the five arguments, should be read with care and with the help of good commentary, such as from the great Dominican scholar Father Reginald Garrigou-Lagrange or the contemporary Dr. Edward Feser.

To the five arguments of St. Thomas may be joined other arguments for God’s existence from different vantage points, such as St. Augustine’s argument from the desire for happiness, to St. Anselm of Canterbury’s famed ontological argument of God as “that than which nothing greater can be conceived,” to Blessed John Henry Newman’s argument from morality.

The point remains for us that while it is commonly thought that reason has nothing to say with respect to the question of God’s existence, it actually has much to say! This question can be engaged from the perspective of reason, as well as from the perspective of faith. While it takes faith to inform us about the higher truths unattainable by reason alone (such as God’s Triune nature and the Incarnation), reason by itself can inform us of a great deal. One thing unaided reason can attain to is certainty of the existence of God. Can we prove that God exists? Absolutely. We just have to open our minds and let in the light of God.

Jamie Cassata is a freelance writer and earned his Master of Theology and Christian Ministry at Franciscan University of Steubenville.